Field of the Invention
The present invention relates to gas separation membranes.
Related Art
Membranes are commonly used for large scale fluid (water or gas) separation processes. Gas separation membranes are commonly manufactured in one of two configurations: flat sheet or hollow fiber. The flat sheets are typically combined into a spiral wound element. On the other hand, the hollow fibers are commonly bundled together in a manner similar to a shell and tube heat exchange or they are wrapped around a mandrel.
In typical spiral wound flat sheet membranes, two flat sheets of membrane with a permeate spacer in between are joined, for example glued, along three of their sides to form an envelope (i.e., a “leaf”) that is closed on three sides but open on one side. These envelopes are separated by feed spacers and wrapped around or otherwise to form a perforated permeate tube with the open side of the envelope facing the permeate tube. Feed gas enters along one side (i.e., the feed gas side) of the wound membrane element in between two adjacent envelopes and passes through the membrane element in an axial direction. As the gas travels between adjacent envelopes, more permeable fluids permeate through one of the sheets and into an interior of the envelope. These permeated gases have only one available outlet (the single open side of the envelope), so they travel within the envelope in an inwardly spiraling path, out the open envelope side, and to the permeate tube. The primary driving force for such transport (from the feed side to the permeate tube) is the pressure differential between the high feed gas pressure and the low permeate gas pressure. The permeate gas enters the permeate tube, such as through perforations formed in the tube. The gases that do not permeate the sheet are referred to as the non-permeate gas (or residue or retentate). The non-permeate completes travel through the spiral wound sheet in the axial direction and exits the side of the membrane element opposite that of the feed gas side.
In hollow fiber elements, very small hollow fibers are laid around a central tube either arranged parallel to the axis of the tube or helically wrapped around the tube. This achieves a fairly high packing density. In one type of hollow fiber membrane, the bores of the fibers at one end thereof are sealed off from the feed gas with a tubesheet at one end of the membrane element. In another type of hollow fiber membrane, the bores of the fibers at each end thereof are sealed off from the feed gas with a tubesheet at each end of the membrane element. Feed gas is fed to the outer circumferential surface of the membrane element and flows over and between the fibers. More permeable gases permeate across the fiber wall into the fiber bores. The permeate gas then travels within the fiber and is collected at the tubesheet(s). When two or more membrane elements are arranged in parallel, the permeate gas from one membrane element is mixed with the permeate gases from the other membrane elements. Typically, the combined permeate gas exits the membrane element through a permeate conduit or pipe. Gases not permeating through the fiber wall eventually reach a central tube of the membrane element, which is typically perforated. While many configurations for the permeate conduit have been proposed, in one such configuration, the central tube is divided into two regions extending throughout the entirety of the central tube. In such a divided region tube, the non-permeate gas is conveyed in the outer region while the permeate gas is conveyed in the inner region. The inner region is sealed off from the outer region but fluidly communicates with a permeate gas collection element formed in the tubesheet.
Some have proposed to arrange multiple spiral wound membrane elements in series within a single pressure vessel, such as U.S. Pat. No. 5,851,267 and U.S. Pat. No. 7,338,601. Gases on the feed side that do not permeate through the flat sheet leave through the non-permeate side of the membrane element opposite the feed gas side. The non-permeate gas from the first membrane element in the pressure vessel constitutes the feed gas for the second membrane element, the non-permeate gas from second membrane element constitutes the feed gas for the third membrane element, and so on. The permeate tubes of each of the membrane elements are aligned to form a single tube that sequentially receives permeate gas from each the membrane elements in the pressure vessel. In these types of membrane modules, in order to maintain the integrity of the of the feed, permeate, and non-permeate gases. Generally speaking, these types of membrane modules have utilized a seal disposed between the feed gas inlet/port and the membrane elements in order to prevent feed gas from leaking into the annular space in between the outer surface of the membrane element and the inner surface of the pressure vessel.
In particular, U.S. Pat. No. 5,851,267 discloses a single seal at the feed end of the pressure vessel and other seals at each membrane-to-membrane element pairing. At the feed end, a ring-shaped flange is attached to the first upstream membrane element (upstream in the context of the direction of the flow of the feed) and the annular space in between the flange and the inner wall of the pressure is sealed with a seal such as an O-ring. Because this single seal is only provided to the first upstream membrane element after all of the membrane elements have been sequentially loaded into the pressure vessel, this single seal does not have to exhibit such a low resistance to sliding that it can be slid down the length of the pressure vessel. The remaining seals in between adjacent membrane elements also do not have to exhibit such a low resistance to sliding that it can be slid down the length of the pressure vessel. This is because each the O-rings in a membrane-to-membrane seals only needs to be slid a very small distance to allow one half of the seal attached to one membrane unit to engage with the other half of the seal attached to the other membrane in the membrane pairing. The O-ring is then secured into place by twisting one of the seal halves with respect to the other seal half until it is locked.
The in-series membrane element arrangement disclosed by U.S. Pat. No. 5,851,267 has some drawbacks. For example, performance of the downstream membrane elements degrades more quickly than that of the upstream membrane elements (upstream and downstream again in the context of the flow direction of the non-permeate). This leads to inefficient operation. Also, the series configuration results in a greater pressure drop from the feed to residue (also known as retentate or non-permeate).
Therefore, it is an object of the invention to provide a membrane module including multiple membrane elements that does not suffer from the above-described drawbacks.
Instead of membrane elements in series, U.S. Pat. No. 4,670,145 discloses a membrane module including several membrane elements in a same pressure vessel that are arranged in parallel instead of in series. In other words, each membrane element receives the feed gas instead of the non-permeate gas from the adjacent, upstream membrane element (upstream in the context of the direction of the flow of feed gas). While the solution proposed by U.S. Pat. No. 4,670,145 is generally satisfactory, it requires access to each end of the pressure vessel.
Therefore, it is another object of the invention to provide a membrane module including multiple membrane elements arranged in parallel that does not require access to each end of the pressure vessel.
U.S. Pat. No. 7,338,601 discloses a membrane module including several membrane elements arranged in series, including one embodiment in which the non-permeate is withdrawn from a middle of the pressure vessel and a seal is provided at each end of the pressure vessel that seals the feed gas from the permeate. Similar to U.S. Pat. No. 5,851,267, the membrane arrangement disclosed by U.S. Pat. No. 7,338,601 also exhibits faster performance degradation of the downstream membrane elements in comparison to the upstream membrane elements (in the context of the direction of the flow of feed gas), leading to inefficient operation. It also results in a greater pressure drop from the feed to residue (also known as retentate or non-permeate).
Therefore it is another object of the invention to provide a membrane module having a non-permeate withdrawal port in the middle of the pressure vessel and membrane elements arranged in parallel instead of in series.